You are exactly right about the thermostats for the zoning. The plan is to have two thermostats, and keep the second zone warmer/cooler depending on the season. Both installers asked for the window specs, door specs, insulation, etc and are figuring that in. One has come back saying his calculations arrived at 3.6 tons, so he'd recommend a 4 ton unit. Still waiting on the other installer. In an interesting aside, the WaterFurnace territory manager called me yesterday. He said he felt a 3 ton Envision unit would most likely do it for me, but did not have all the figures to make an accurate call, non-committal, etc. We're not typical heat/air users. We keep it warm in the summer and use ceiling fans, cool in winter and use a woodstove.

Follow-up: I got a full audit with blower door test, which was awesome. Found a number of prominent leaks, some of which I'm plugging myself (supply registers, outlets, windowframes), some of which are for future consideration (basement), and some were plugged when I got my attic sealed and insulated yesterday. It turned out that after my recent roofwork, there were some areas of the attic that really needed to be sealed and insulated, so I am I happy I got that done in any case. Now we will see how the house performs on a hot day, assuming we ever get another one around here. Next on my todo list is probably my horrible supply ducts in the basement for the upstairs supplies (undersized, uninsulated and leaky). But we'll see how things go in the meantime.

I have a walk-up attic (900 sqft) that I finished off a few years ago. Half the space is for the TV, the other half a game room for the kids...ping pong, fosball, etc. I'm in CT, it was a typical attic space, open framing with FG batt insulation...hot in the summer (110-120 degrees), cold in the winter (50-55 degrees). When I finished it off, I used sheets of 2" thick polyisocyanurate foil-faced foam insulation on the ceiling rafters, and 1" polyiso board on the gable end walls. I installed furring strips 16" on center over the foam insulation, then drywall was screwed to the furring strips. I have radiant floor heat in the rest of the house and was going to add two loops to the system to warm the attic. The two additional loops were planned for in the oiriginal plans when I designed the house. Ended up not having to install the loops. Winter temps the attic gets to a low of about 66-68 degrees. As soon as anyone goes up there or turns on the lights, the room warms up a few degrees. During kid parties or sleepovers, they open the windows to cool the attic down, even in winter. The polyiso is a great radiant barrier, stopping heat gain in the summer. High summer temp up there is about 77 degrees. Since the air handler for the second floor of the house is behind the attic kneewall, I did add a few supply and return outlets to move air and to keep the attic from getting stuffy in the summertime. At R-7 per inch, polyiso is quite effective. Better than that, it stops air infiltration and eliminates drafts, resulting in an even more effective R-value than when compaed to fiberglass. It also is an effective radiant barrier. Cellulose would be my second choice for insulating an attic. Best, Mongo

There are many ways to 'design' a zone system. Some ways are less efficient than others. In most cases, when it comes to these type of zone systems the designs are such to make it easier to install --- but not for the best in efficiency. In order for the zone system to be most efficient, you have to have serious control knowledge. There are a plethora of zoning controls out there. Some are very poor choices. The better controls require good 'design' practices or the zone system will likely trip out with faults or run erratically. The biggest enemy of a zone system is too much air pressure. To alleviate this pressure some manufacturers will keep dampers slightly open. Part of this is to reduce cost so that you can skip the bypass damper. The problem is these methods don't make the system as efficient as it could be. It all comes down to 'design' as well as the controls chosen. If you choose the right controls and the 'design' is wrong... It won't work very good. With that said, a properly zoned system with dampers that close solidly will supply air to the zones on occasion. Typically, this occurs when the system first starts up because all zone dampers usually start from an open position and take a few seconds to fully close.